Method for cleaning tar-bearing waste water and apparatus for performing said method

ABSTRACT

In a method and an apparatus for cleaning tar-bearing waste water ( 17 ), a mixture of water and hydrocarbons, e.g. comprising polyaromatic hydrocarbons and phenols, the mixture is separated into a low-boiling-point part and a high-boiling-point part, bringing the low-boiling-point part on vapour form in a boiler ( 1 ), and the low-boiling-point part is cracked in vapour form at a high temperature in a reactor ( 2 ), providing light combustible gases, which can be utilised in e.g. gas engines, gas turbines or the like. Furthermore, the high-boiling-point part may be used for energy supply to the process or other processes or as an alternative be cracked for providing light combustible gases.

TECHNICAL FIELD

[0001] The present invention relates to a method for cleaningtar-bearing waste water and an apparatus for performing the method ofthe kind set forth in the preamble of claims 1 and 9, respectively.

BACKGROUND ART

[0002] In gas production, based on gasification of biomass, coal, etc.,it is well-known that the produced gas contains tar, which in connectionwith the use of the gas in e.g. internal combustion engines or gasturbines, has to be removed from the produced gas. The traditional wayof removing the tar is by cooling the gas, whereby the tar and possiblewater is condensed, leaving a relatively clean gas and a separatedmixture of water and tar.

[0003] The mixture of water and tar cannot be disposed of to theenvironment due to the content of tar, including polyaromatichydrocarbons and phenols, which are considered environmentally unsafe,possibly carcinogenic, poisonous, etc. Furthermore, the acidity of themixture may also constitute an environmental problem.

[0004] It has been suggested to use ultraviolet light-induced wetoxidation or adsorption on various coke sorbents to clean the wastewater. However, electric energy consumption is relatively high for theultraviolet light-induced wet oxidation and the adsorption has certainlimitations due to the presence of non-adsorbable compounds.

[0005] It has been suggested (Swedish application 402214) that the wastewater is evaporated and separated into a combustible tar fraction and a(more or less) clean steam fraction. The combustible fraction issubsequently burned (oxidised) to provide heat for the evaporationprocess—either by direct contact or through a heat exchanger. However,in this way the calorific value of the tar is converted directly toheat, which severely limits the utilisation in high efficiency powerproducing machinery—specifically gas engines and gas turbines.Therefore, the application of the technology suggested will be limitedto steam turbines and other equipment based on external firing.

DISCLOSURE OF THE INVENTION

[0006] It is the object of the present invention to provide a method forcleaning tar-bearing waste water and an apparatus for performing saidmethod of the kind referred to above, with which it is possible toprovide a high efficiency of the process. By high efficiency is meanthigh cleaning capability and also efficient recovery of the energeticcontent in the tar contaminants for direct use in a gas-engine or gasturbine. This also implies, that in contrast with Swedish application402214—where the contaminants are oxidised—the present inventiondescribes a cracking process conducted under reducing conditions, wherethe high molecular tars and acids are transformed into light combustiblegases which can be utilised in e.g. gas engines, gas turbines or thelike. This object is achieved with a method for cleaning tar-bearingwaste water and an apparatus for performing said method of said kind,which according to the present invention also comprises the features setforth in the characterizing clause of claims 1 and 9, respectively. Withthis arrangement, a relatively clean part of the waste water on vapourform is cracked at a high temperature providing energy containing lightcombustible gases for use in e.g. gas engines, gas turbines or the like,and at the same time a high-boiling-point part of the waste water isprovided, which is a combustible concentrate of the waste water, whichmay be used for heating purposes, e.g. as stated in claims 5 and 15,respectively. Preferred embodiments of the method and apparatus arerevealed in the subordinate claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] In the following detailed part of the present description, theinvention will be explained in more detail with reference to theexemplary embodiments of an apparatus for performing the method inaccordance with the present invention shown in the drawings, in which

[0008]FIG. 1 schematically shows an apparatus in accordance with thepresent invention, and

[0009]FIG. 2 schematically shows a flow scheme of an alternativeapparatus in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The apparatus shown in FIG. 1 is suited for cleaning tar-bearingwaste water, e.g. coming from a biomass gasification unit, i.a. asdescribed in EP-A-953,627. The apparatus comprises an evaporator 1receiving the tar-containing water 17 at the bottom for completelyevaporation in the evaporator 1. Before leaving the evaporator 1, ahigh-boiling-point part is separated by means of a droplet separator 7,said high-boiling-point part being pumped away by means of a concentratepump 8. The low-boiling-point part is moved onwards by a high-pressurefan 6 feeding a heat exchanger 3, in which the low-boiling-point part isheated up to a relatively high temperature before entering the reactor2, in which the low-boiling-point part is cracked in order to reduce itscontents of hydrocarbons to an environmentally acceptable level, saidcracking reactor 2 comprising a burner 5 supplied with concentrate fromthe pump 8 and combustion air 16, thus heating up the reactor 2 byburning off at least part of the concentrate separated out by means ofthe droplet separator 7. The cracked product leaving the reactor 2 isfed downwards in the heat exchanger 3 whereby it is cooled down by theheat exchange with the low-boiling-point part in counterflow. The cooleddown cracked product 4 leaving the heat exchanger 3 is split up into twoportions, one fed to the evaporator 1 providing heat for evaporation ofthe tar-containing water, whereby this part of the cracked product 4 iscondensed and leaves the evaporator 1 in the form of relatively cleanwater 18, sufficiently clean to be led to the sewer system. Another partof the cracked product 4 leaving the heat exchanger 3 can be used asgasification medium in the associated gasifier as indicated at 20.

[0011] An alternative apparatus in accordance with the invention isshown in FIG. 2 in the form of a flow scheme, in which partscorresponding to the parts shown in FIG. 1 are supplied withcorresponding numbers. The apparatus shown in FIG. 1 again comprises anevaporator 1, which is supplied with tar-containing water 17 and whichevaporates this tar-containing water using a closed loop of pressurisedhot water for the heating, said hot water being heated by means ofexhaust 10 from one or more engines, e.g. driven by gas from a biomassgasification plant, said heat being extracted from the exhausted gas inan exhaust boiler 9, said heating being supplemented by heat extractedfrom the cracked vapour 4 leaving the heat exchanger 3, extracting saidheat in a separate heat exchanger 12. The evaporator 1 again ends up ina separator 7 separating out a concentrate which is delivered to aconcentrate buffer tank 14 from which it can be pumped up to a burner 5for this concentrate by means of a concentrate pump 8. Thelow-boiling-point part leaving the evaporator 1 after the separator 7 issubjected to heat exchange with cracked vapour from the reactor 2 in aheat exchanger 3, said heat exchanger 3 further comprising separatechannels for pre-heating air 15 for use in the reactor 2. The reactor 2is heated by means of the burner 5 supplied with air 16 for burning theconcentrate from the buffer tank 14 and further heat is supplied byparts of the low-boiling-point part burning when coming into contactwith the preheated air 15 in a first part of the reactor 2. The hightemperature in the reactor 2 provides a cracking of the mixture suppliedthereto and the high temperature cracked vapour leaving the reactor 2supplies heat to the low-boiling-point part from the evaporator 1 andthe air 15 for the reactor in the heat exchanger 3. The cracked vapour 4leaving the heat exchanger 3 is supplied to the heat exchanger 12 inorder to deliver energy to the hot water pressurised closed loop forheating the evaporator 1. After leaving the heat exchanger 12 the vapourcan furthermore deliver energy to a district heating loop 19 using aheat exchanger 13 in which parts of the cracked vapour may condensateand other parts, e.g. non-condensable, light combustible parts of thecracked product from the reactor 2, are fed to a gas engine or gasturbine 22.

[0012] In a preferred process in the system shown in FIG. 2, the hotwater pressurised loop extracts heat from the engine exhaust 10 in theexhaust boiler 9, whereby the water is heated up to a temperature ofabout 120° C. and this heat is transferred to the incomingtar-containing water 17 in the evaporator 1 resulting in vapour at thetop of the evaporator at a temperature of about 107° C. In the separator7, a liquid concentrate is separated and collected in a concentratebuffer tank 14 leaving a relatively clean steam, which is heated in theheat exchanger 3 to a temperature of about 450° C. in counterflow withcracked product and combustion gases leaving the reactor 2. In the heatexchanger 3, air or an inert gas 15, which may be preheated by passingthrough the mantle-cooling conduct for the heat exchanger 3, is heatedto about the same temperature as the steam, when entering a first partof the reactor 2. The inert gas could be provided in the form of cooledexhaust gas 11. The possible hot air spontaneously ignites some of theorganic compounds in this first part of the reactor 2 and further heatis added in order to raise the temperature in the reactor to about 800°C. The energy for this is provided by burning concentrate from theconcentrate buffer tank 14 in the burner 5.

EXAMPLE

[0013] In connection with the cleaning of tar-containing water inconnection with a gasifier, the following cleaning capabilities apply:

Typical Inlet Conditions

[0014] Acetic acid 14,200 mg/litre

[0015] formic acid 1900 mg/litre

[0016] causing an acidity of pH=2.03

[0017] phenol 730 mg/litre

[0018] guaiacol 1030 mg/litre

[0019] dehydroxy-benzen 1 1400 mg/litre

[0020] other phenols 2840 mg/litre

[0021] and further the PAH's:

[0022] naphthalene 0.45 mg/litre

[0023] antracene/pheantrene <0.005 mg/litre

[0024] the total organic carbon content (TOC) is 45,900 mg/litre

Typical Exit Conditions

[0025] The original contaminated water is separated into two streams:

[0026] A heavily contaminated fraction (about 10%) with a TOC of about300,000 mg/litre and gross calorific value of about 13 MJ/kg (65-75% ofthis will be reused internally in the process—the rest may be burned inauxiliary boilers in the plant during district heating peak loads).

[0027] A clean condensate with a TOC below 15 mg/litre, a total phenolcontent below 0.15 mg/litre and an acidity of pH=6.90 to 7.10 (therebyeliminating the need of neutralisation).

EXAMPLE 2

[0028] In a plant corresponding to FIG. 1, 1266 kg/hour of waste wateris boiled in the evaporator 1 heated on the outside using clean steam 4at a temperature of about 550° C. and a pressure of 102 kPa leaving theheat exchanger 3. The waste water leaves the evaporator 1 as:

[0029] steam 1152 kg/hour at about 97° C. and 100 kPa, which after thedroplet separator 7 is compressed in a high-pressure fan 6 to about 105°C. and 105 kPa, a combustible concentrate 114 kg/h of which the majorfraction is used in the process at the burner 5 as described above. Thesteam part is heated in the heat exchanger 3 in counterflow with steamleaving the reactor 2 to about 380° C. and 104 kPa. After the heatexchanger, the temperature is raised to 800° C. using about 81 kg/h ofthe combustible concentrate burned off in the burner 5. Based on 0.305kg/MJ, about 320 kg/h air 16 is used at this point. In the FIG. 1apparatus, the burner 5 burns directly Inside the reactor 2 therebypromoting turbulence and elimination of remaining tar traces in thesteam. The steam leaving the reactor 2 will have a flue gas content ofabout 22%, which will reduce the performance of the steam-heatedevaporator due to the presence of inert gases. These inert gases willhave to be withdrawn from the top shell part of the evaporator 1 inorder to improve condensation heat transfer to the evaporator. Thepurified water 18 leaving the evaporator 1 amounts to about 90% of thetar-containing water input 17.

[0030] The purified water has a TOC of about 14 mg/litre and containsabout 0.4 mg/litre phenol, where regulations require below 15 mg/litre.

[0031] Although the invention above has been described in connectionwith preferred embodiments thereof, it will be evident for a man skilledin the art that several modifications are possible within the scope ofthe following claims.

EXAMPLE 3

[0032] By operating the evaporator in a way to admit a larger fractionof organic contaminants in the steam from the evaporator oralternatively (or additionally) inject part of or all of the separatedtar-contaminants from the separator directly into the reaction chamber,which is being held at low stoechiometric conditions, the tar componentswill be cracked into lighter combustible gases. When the tar-watercleaning system described is used in connection with a gasifier thesegases might subsequently be added to the gas cleaning system of theassociated gasifier and increase overall power efficiency.

1. Method for cleaning tar-bearing waste water (17) (a mixture of waterand hydrocarbons, e.g. comprising polyaromatic hydrocarbons andphenols), characterized by comprising a) separating the mixture into alow-boiling-point part and a high-boiling-point part, bringing thelow-boiling-point part on vapour form, b) cracking (2) the highmolecular hydrocarbons from the low-boiling-point part in vapour form ata high temperature, thereby converting its contents of hydrocarbons toproducts that are on vapour form at atmospheric pressure and ambienttemperature, which products are light combustible and can be utilised ine.g. gas engines, gas turbines or the like, c) condensation of the watercontents of the cracked low-boiling-point part, thus separating thecracked low-boiling-point part in water and light combustiblehydrocarbons.
 2. Method in accordance with claim 1, characterized byfurther comprising d): cracking the high molecular hydrocarbons e.g.polyaromatic hydrocarbons and phenols from the high-boiling-point partat a high temperature, thereby converting its contents of hydrocarbonsto products that are on vapour form at atmospheric pressure and ambienttemperature, which products are light combustible and can be utilised ine.g. gas engines, gas turbines or the like.
 3. Method in accordance withclaim 1 or 2, characterized by the separation being performed by a1)completely evaporating the mixture and a2) condensation and separationof the high-boiling-point part.
 4. Method in accordance with claim 1 or2, chacacterized by the separation being performed by a3) portionwiseboiling off the low-boiling-point part of the mixture, leaving thehigh-boiling-point part as a residue.
 5. Method in accordance with anyof the preceding claims, characterized by comprising the burning of atleast part of the high-boiling-point part for providing the hightemperature for the cracking processes under b).
 6. Method in accordancewith claim 5, characterized by comprising the burning or cracking of atleast part of the high-boiling-point part directly in the vaporised partto be cracked.
 7. Method in accordance with claim 5, characterized bythe burning of the high-boiling-point part providing an indirectlyheating of the vaporised part to be cracked.
 8. Method in accordancewith any of the preceding claims, characterized by further comprising aheat exchange (3) between the cracked vapour and the vaporised lowboiling-point part to be cracked.
 9. Method in accordance with any ofthe preceding claims, characterized by at least part of the crackedvapour (4) being used to provide energy for the evaporation of thetar-containing water.
 10. Apparatus for cleaning tar-bearing waste water(17), e.g. a mixture of water and hydrocarbons, e.g. polyaromatichydrocarbons and phenols, characterized by comprising a boiler orevaporator (1) for evaporation of the mixture, a separator (7) forseparating the high-boiling-point part of the mixture and a reactor (2)for cracking the remaining evaporated mixture, a heat exchanger (3)between the boiler (1) and reactor (2) for performing a heat exchangebetween the mixture flowing towards the reactor (2) and the mixtureleaving the reactor (2), and a connection (4) feeding the mixtureleaving the reactor (2) and heat exchanger (3) to the boiler (1),thereby extracting energy for the evaporation from this mixture. 11.Apparatus in accordance with claim 10, characterized by furthercomprising means for controlled adding part of the separatedhigh-boiling-point part to the reactor (2).
 12. Apparatus in accordancewith claim 10 or 11, characterized by comprising a separate reactor forcracking the high-boiling-part of the mixture.
 13. Apparatus inaccordance with any of the claims 10-12, characterized by the reactor(2) comprising a burner (5), in which the separated high-boiling-pointpart is burned for supplying heat to the reactor (2).
 14. Apparatus inaccordance with any of the claims 10-13, characterized by furthercomprising a high-pressure fan (6) for increasing the pressure in thereactor (2), and/or decreasing the pressure in the boiler (1), said fan(6) preferably being positioned at the outlet from the boiler (1). 15.Apparatus in accordance with any of the claims 10-14, characterized bythe separator (7) being formed by a droplet separator (7) at the outletfrom the boiler (1).
 16. Apparatus in accordance with any of the claims10-15, characterized by the reactor (2) being fitted with a component,which functions as a catalyst to the cracking, process.
 17. Apparatus inaccordance with any of the claims 10-15, characterized by the reactor(2) being formed of a high-temperature steel alloy containing nickel,which nickel functions as a catalyst to the cracking process. 18.Apparatus in accordance with any of the claims 10-16, characterized bythe reactor (2) being lined with high-temperature refractory.